The present invention relates to a method for eliminating interference in a measurement signal which contains so much noise that it is necessary to smooth it to obtain the measured value, where the interference includes elements which would lead to a systematic error in the formation of the measured value.
In particular, the present invention relates to a method and a circuit arrangement with which interference in measurement signals can be detected on the basis of the time behavior of the interference and such interference can thus be suitably considered when a measured value is formed from the measurement signal, for example for the determination of blood circulation in the skin.
Analog measurement signals are subject to interference from very different causes. Systematic errors unilaterally change a measured value and are thus difficult to detect and estimate.
Statistical interference, or noise, can be reduced or completely eliminated by integration over time. However, it must then be assumed that the measured value remains unchanged during time intervals corresponding to the magnitude of the time constant employed. Generally, the constancy over time of a measured value imposes a limit on the value of the time constant, and thus limits the measuring accuracy. Reduction of statistical errors by integration over time is, as a practical matter, the result of a limit frequency being introduced into the spectrum of the measurement signal. The components above the limit frequency are considered interference and are eliminated by a frequency filter. The separation between interference and the instantaneous measured value is thus made on the basis of spectral characteristics.
However, separation with the aid of a frequency filter presupposes that the interference results in changes in the measurement signal, with the frequency and amplitude of such changes being distributed symmetrically around the true measured value.
However, often this is not the case. If, for example, a measurement signal containing a considerable amount of noise, which must therefore be filtered through a lowpass filter and which has a relatively slowly changing average, occurs together with comparatively short interferences which exclusively increase, or decrease the magnitude of the instantaneous measurement signal by a substantial amount, the measured value obtained by integration over time is systematically increased, or decreased. Thus, in this case, integration over time results in an additional systematic error which can no longer be detected due to the integration and lowpass filtering, respectively.
Equivalent interferences also occur in a digital signal which is formed, for example, of a sequence of unit pulses and in which the measurement signal corresponds to the rate of the unit pulses. An example is the measurement of light intensity by means of a photomultiplier which is operated as photon counter. The occurring counting rate for the photon pulses is proportional to the light intensity. Particularly for the determination of low intensities, which is possible only with long measuring interval lengths, there occur a whole series of interferences. One type of interference is characterized by the fact that additional unit pulses are generated in a relatively small time interval and thus the counting rate is systematically increased. This of course falsifies the measured value for the light intensity to be measured. The cause of such so-called "bursts" is electromagnetic interference, cosmic radiation and radioactivity in the environment (specifically K-40 in the glass of the photomultiplier).
For measurements at low counting rates it is therefore necessary to operate, in addition to the measurement photomultiplier, an identical second control photomultiplier equipped with photon counting electronic system on which no light impinges. Occurrence of a burst in the control photomultiplier indicates interference. Further processing of the pulses from the measurement photomultiplier is then interrupted.
Particularly when such bursts are caused by cosmic radiation, there arises the difficulty that the control photomultiplier should really be located at the same place as the measurement photomultiplier. This is accomplished in that the actual measuring photomultiplier is surrounded by a plurality of control photomultipliers and then, if there is coincidence in the control photomultipliers, further processing of pulses from the measuring photomultiplier is interrupted. Consequently, this process is very expensive.
A further possibility of reducing the influence of the bursts is to disable the counter for a certain time interval after it records a pulse, i.e to provide an extended dead time. This would cause pulses appearing in rapid succession to not be registered. However, since the duration of a burst may lie several orders of magnitude above the average time interval between two successive photon pulses, the dead time must either be made too long or interference pulses would again be recorded at the end of the dead time and would likewise falsify the measured value.
Analog signals may also be subject to interference in that relatively high amplitude values occur within relatively short time intervals.
A measurement signal of this type occurs, for example, in a device disclosed in earlier FRG patent applications OS No. 3,242,771 and OS No. 3,401,535. Here, a laser optical method is employed to measure blood circulation in the skin. Since, however, the major portion of the laser light is scattered at the epidermis, through which no blood flows, and does not penetrate to the depth of the layers in which there is circulation, two signals are superposed on one another, these signals being derived, respectively, from: the movement of the blood; and that of the epidermis. Originally, the signal from the movement of the epidermis is predominant. This influence can be reduced by various measures but it cannot be suppressed completely.
Such a signal is shown in FIG. 7a of the accompanying drawing, where blood circulation through the skin (value VA) changes only slowly so that it can be considered to be constant within the scope of the discussion here. The noise superposed on this blood circulation value is caused primarily by the speckle structure on which the measurement is based, as well as, inter alia, by the laser and the electronic system.
The occurring peaks P.sub.1 to P.sub.7 are caused in that there is intentional or unintentional movement of the skin at the point of measurement. These are sometimes brief twitches. Particularly patients suffering from circulatory malfunctions in the extremities, where measurements of blood circulation in the skin are made, are afflicted with so-called test pains which they can alleviate by intentional or unintentional twitching.
To reduce noise, the signal MA can be filtered through a lowpass filter. The time constant of the lowpass filter must be dimensioned such that the occurring frequencies in the noise are noticeably attenuated. Since the spectrum of the interference pulses lies in the same frequency range, a modified signal ZT appears at the output of the lowpass filter, as shown in FIG. 7b. The noise is visibly reduced but the interference, due to its magnitude, cause the resulting signal to rise rapidly and decay slowly according to the time constant. If the time constant is increased, the rise at the occurrence of interference is less steep but the time until the signal returns to the level of the measured value becomes longer. In the example given here, an overly high value is indicated during a considerable portion of a measurement interval having a duration T.
Another possibility of reducing the noise is the integration of MA with corresponding time constants during a measuring interval of a length T. This also does not eliminate the interference but causes it to form steps in the curve ZI shown in FIG. 7c. The errors are summed so that at the end of the measuring interval, a value is indicated which is excessive compared to the actually measured value. The broken line in FIG. 7c shows the form which curve ZI would have if no interference occurred.